1. Field of the Invention
The present invention relates to a discharge tube lighting apparatus for lighting a discharge tube, such as a cold-cathode tube for use in a backlight in, for example, a liquid crystal display.
2. Description of the Related Art
A typical induction motor and inverter that drives a high-intensity discharge (HID) lamp adjusts its output power by controlling a peak value. Japanese Unexamined Patent Application Publication No. 6-105563 discloses a circuit that includes a power-factor correction (PFC) converter that performs pulse-amplitude modulation (PAM) control during heavy loading and pulse-width modulation (PWM) control during light loading to expand a range of controlling the output power and an inverter receiving the output of the converter and driving an induction motor. Japanese Patent No. 3752222 discloses a circuit that includes a PFC converter and an inverter that receives the output of the converter and drives a HID lamp.
A power supply for a backlight in, for example, a liquid crystal display is required to supply power in a wider range of power supplying a power to an inverter than that for an induction motor or an HID lamp. This is because the backlight is usually used in a dark room with a low luminance and it is necessary to increase the luminance of the backlight in a bright room. If that control is made by PWM, a reduction in peak value of a voltage input to the inverter or a voltage distortion (a phenomenon in which the voltage deviates significantly from a sine wave shape) may occur when the luminance is low. This may cause the backlight to flicker or the backlight may not be properly lit. To address this problem, burst control as disclosed in Japanese Unexamined Patent Application Publication No. 11-122937 is used.
Here, the backlight control device illustrated in Japanese Unexamined Patent Application Publication No. 11-122937 is described with reference to FIG. 1.
In FIG. 1, a current passing through a fluorescent lamp 4 is detected by a resistor R4 as a voltage signal and is rectified by a diode D1 and a capacitor C3, and a mean voltage is extracted. The mean voltage and a dimming voltage Vcon are divided by a resistor R1 and a resistor R2 and input to a dimming control circuit 1. The dimming control circuit 1 outputs an on-off signal to duty-control a transistor Q1 at a frequency in the range of from a fraction of to several-tenths of an oscillating frequency of the inverter circuit, using the input voltage, and the transistor Q1 controls the voltage to be input to the inverter circuit. That is, when the dimming voltage Vcon decreases, the voltage input to the dimming control circuit 1 decreases, such that the dimming control circuit 1 operates so as to extend the period in which the transistor Q1 is in an ON state and to increase the length of the period in which a current passes through the fluorescent lamp 4. In contrast, when the dimming voltage Vcon increases, the voltage input to the dimming control circuit 1 increases, such that the dimming control circuit 1 operates so as to reduce the period in which the transistor Q1 is an ON state and to reduce the length of the period for which a current passes through the fluorescent lamp 4. The ratio between the period in which the fluorescent lamp is illuminated and the period in which the fluorescent lamp is not illuminated at that time changes the intensity of the backlight.
The voltage to be input to the inverter circuit is extracted as a voltage divided by a resistor R5 and a resistor R6, and the detected voltage is input to an input-voltage control circuit 2. The input-voltage control circuit 2 outputs an on-off signal to duty-control the transistor Q1 at a frequency that is twice the oscillating frequency of the inverter circuit, using that input voltage, and the transistor Q1 limits the voltage to be input to the inverter circuit to a preset value.
The on-off signals output from the dimming control circuit 1 and the input-voltage control circuit 2 are ORed by a logic circuit 3, thereby allowing the transistor Q1 to perform burst control and PWM control.
However, in the backlight control device disclosed in Japanese Unexamined Patent Application Publication No. 11-122937, because of the effects of the burst operation of the converter at a previous stage, the input current is a pulse current. Thus, if a circuit including only Q1 and L1 at a previous stage is a PFC converter, when the tube current of a cold-cathode tube that is a load is fed back and the output voltage of the PFC (voltage to be input to the inverter circuit) is subjected to burst control, the tube current would also be reduced and the PFC would not normally operate during the period in which the inverter circuit is inactive, so the power factor would be degraded.
Recently, there is a trend in liquid crystal televisions and other products to drive a cold-cathode tube lighting apparatus requiring a relatively high voltage and other loads, including a central processing unit (CPU), using a shared power supply circuit. However, if the converter is subjected to burst control, the entire circuit is inactive when the converter at a previous stage is inactive due to the burst control. Therefore, a problem arises in which the output voltage of the converter at the previous stage can be used only in an input to the inverter.